Burns of engines with poor vaporization can be as long as 45 degrees. THAT'S WHY some engines make best power at timings like 38 to 45 degrees compared to another that best at 28 degrees.

The burn time determines the peak cylinder pressure.

Can you explain this further and how initial timing plays in engines with poor vaporization? Reason i ask is my smallblock mopar 360 with 256 @ .50 cam 108 lc with closed chamber heads and 185 psi cranking comp seems to want 20-25 initial timing. I would have thought that this type of chamber would be more efficient and therefore require less initial? Can a lean idle require more initial timing to burn right?

i assume your talking idle timing as being 20 to 25 for best idle running on your engine. Lean mixture requires more timing advance. Lean burns slow. The other thing that slows burns is exhaust gas contamination of the fresh charge. So think about what 108 lsa does compared to say 114.The relationship of vaporization to timing is thus. With equal fuel for comparing purposes of 2 engines, one with poorly vaporized mixture the other with very good vaporization, the engine with poor vaporization will require energy that is released from the combustion to bring it to an equal state of vaporization compared to the very good engine. You can think of its in as simple terms as that and you wont be wrong in any conversation. Any energy removed from the combustion to achieve further combustion is wasted and non productive at the crank.

i assume your talking idle timing as being 20 to 25 for best idle running on your engine. Lean mixture requires more timing advance. Lean burns slow. The other thing that slows burns is exhaust gas contamination of the fresh charge. So think about what 108 lsa does compared to say 114

Yeh it seems to like 20-25 is that normal? Do i need to run it richer at idle to compensate for the excess exhaust residual?

I think that is what im fighting most on my combo. I recently changed from triple baffle mufflers which i needed for engineering and rego to Hookers aero chambers to take out some of the back pressure and it seems better so far. I originally ran it with 16-18 initial and it ran hard but now it seems it wants more after a few changes.

LOL on another note hows the weather.....................1 week we get bush fires and the next floods!

You must be in Australia, where about? If you enrich the mixture at idle it will require less timing and the volume of gas expelled from the chamber will be greater (because there is more fuel in the cylinder ). The greater volume exhausting resists reversion backwards up the exhaust port therefor it actually runs with a cleaner chamber. Funny stuff isn't it. BUT what you have to do is enrich it correctly with the right processes in the carburetor, sometimes it's not just a matter of winding out the idle screw or changing the IFR. Every engine has a design to it and Chrysler's are a bit different to the other US products, they are not chevs or fords, they are better. They have better combustion and its not necessary to follow the rules of the hotrod industry, as the industry is very Chev orientated. Now what i just said should get plenty of people jumping up and down( hopefully in fun).

Yeh im in Sydney and its pissing down atm, so much for the long weekend.

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If you enrich the mixture at idle it will require less timing and the volume of gas expelled from the chamber will be greater (because there is more fuel in the cylinder ). The greater volume exhausting resists reversion backwards up the exhaust port therefor it actually runs with a cleaner chamber. Funny stuff isn't it.

Ive pulled the jetting back about 4 jets front and back. Im running a 3310-2 with stock metering blocks. So i have to run it really rich to improve the idle............ive been trying to lean it out to get rid of that rich idle smell and clean the plugs as they're black.

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BUT what you have to do is enrich it correctly with the right processes in the carburetor, sometimes it's not just a matter of winding out the idle screw or changing the IFR. Every engine has a design to it and Chrysler's are a bit different to the other US products, they are not chevs or fords, they are better. They have better combustion and its not necessary to follow the rules of the hotrod industry, as the industry is very Chev orientated. Now what i just said should get plenty of people jumping up and down( hopefully in fun).

Im all eyes Shrinker! Any thing to do with the long rod ratio and small ports?

Yep Chryslers have better rod ratio, better ports, and better lifter to push-rod angle. The best feeling of hotrod engines comes from Chrysler ones. Hp for Hp they feel better and they have better emissions.I don't know why you would get black plugs with a normal 12.8 AFR idle mixture, perhaps you haven't got that right. You also have to work the correct relationship of t slot to idle port, forget all this junk about drilling butterfly blades, you just file the slot or plug it. Its not for the faint hearted but that the way I do it. Never have a drilled blade, it doesnt work as nice as actually working the slot correctly.

Well when you get AFR info it should help you. I can tune without a gauge but it's very difficult to tell how to do that on the net, and it takes some skill to do, skills that you accumulate over time.

What do you mean expand upon my comment on chevs, what is there to say? Chevs are like VHS verses Beta video, like IBM verse Mac, one is popular the other is better engineering.

LOL.... When you say follow the rules of the hotrod industry what exactly do you mean,how do you approach making power with a chrysler differently to a chev?

Hysteric

Ohh Ok, what i mean is you dont have to do much to make power from the factory design. Chevs have to be hotted up with aftermarket heads to get close a Chrysler. I'm sure some chevs heads must bite soon. Chryslers can have a cam change and extractors and thats all you need to make a powerful car. The cams dont need to be as big as a chev because you dont have to get around that rediculous pushrod pinch caused by the stupid unneeded 5 head bolt arrangement thats a throwback to the 50's way of thinking.

The problem in Australia is they had terrible management here, they gave us nothing. 2 barrel carbys and auto box as the only option for a V8, How stupid is that. People here dont see them as a performance option. Chrysler heads need to get together and spank everyone.

Are you talking stock heads or racing ones/ It doesnt matter from the head point of view as once you go to racing heads then its just a racing head like any other. Stock heads; well you just flow check em and go from there BUT the big thing about Chrysler motors is there stroke rod ratio. They have the theoretically correct 1.85 or get close to it most designs. Fords and Chevs are short ratio engines (unless your using racing blocks etc).

When presented with a short rod engine you dont have many piston to valve clearance issues because the pistons gets pulled out of the way quickly, go to a long rod motor and its a different story, you have to have the right design of piston. This is how I prioritise engine design. 1 get a head that flows the air you need for the power you want. Get a good head, one without turbulence, I hate turbulence it wrecks the fuel preparation. 2 Get the right bore to fit the valves, dont shroud the valve with too small a bore have the modern chamber shape and size it so that the chamber walls blend to the bore. Ive had great difficulty with that one, trying to get the heads machined the way i want them. 3 Select the fuel your going to use, use racing fuels for racing. Fuel selection is key, you have to consider it from the point of view of HP per cube also its not just about compression ratios. 4 Set the compression pressure you need to vaporise that fuel efficiently. 5 Set the stroke so that the piston can be dished with a mirror of the head. Of course this step depends on the compression ratio but Im talking a racing engine here with some pop in it like 13:1 plus. If the stroke and comp ratio doesnt require a dish but requires a dome then make the piston custom fit the chamber so that the squish band ramp to the dome is close to the edge of the chamber, thus pushing the ejected gases around the chamber roof, cleaning the chamber.

With the Chrysler engine having a long rod ratio the piston dwells for longer at TDC so the opportunity for a dished piston is there for a different reason than a Chev etc. Dishing the piston gives a better chamber because the flame kernel is further way from metal in every direction so thats good. It makes the plug more central when viewed from the side if you get what I mean. But the reason for doing it with a Chrysler is different to a Chev. Doing it in the Chrysler long rod engine is not just that the piston clearance is there but its to create a longer flame path before the flame extinguishes against a surface while the rod is dwelling at TDC. 1.8 stroke rod ratio motors are different gas production to shorter rod ratio motors. They have more complete burn with less HC wastage. they are awesome to work with, unfortunately theres not much Chrysler racing stuff here in Oz.

Now the cam selection, Its tied up with the amount of room you have isnt it. So the type of head and its deck to valve seat height matters a lot. The cam you run and its figures is basically something to shut the valve so you get enough compression pressure for the fuel you need and make sure its shut before it starts backflowing up the intake. The simulation progs are handy for that. That single are is the most common mistake I see, too late on closing intake is a bad idea. I dont give much of a stuff about the exhaust, its the intake that people get really wrong. If your going to fix only one thing, fix the intake.

Its easier to start with a small cam and work out how the engine is making its power (look at it gas production) than to try and figure out a monster profile with all sorts of issues. Once the engines personalty is apparent, then a more optimised cam can be "guessed" at Ha HA hA